This application is based upon and claims benefit of priority of Japanese Patent Application No. 2002-149293 filed on May 23, 2002, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a direct current motor having commutator segments firmly embedded in an insulation bond, and more particularly to such a direct is current motor for use in a starter for cranking an internal combustion engine.
2. Description of Related Art
A direct current motor has been used for driving various kinds of devices, and it is always required to make the direct current motor compact in size. Taking a direct current motor for use in a starter for cranking an internal combustion engine as an example, it has been required to make the direct current motor small in size and to increase its rotational speed. To make the direct current motor small in size, all the components constituting the direct current motor, including a commutator, have to be made small in size.
A commutator composed of plural commutator segments circularly arranged and molded together with thermosetting resin such as epoxy resin is commonly used in a starter motor. Since the commutator connected to a rotor rotates at a high speed, it is necessary to provide the commutator with a sufficient mechanical strength to endure a high centrifugal force. The commutator segments simply bonded with molding resin are not strong enough to endure a high centrifugal force. An individual commutator segment may be separated from the molded commutator due to a high centrifugal force applied thereto when the commutator is rotated at a high speed.
In order to increase the bonding strength of the commutator segments, various proposals have been made. For example, it has been proposed to form a claw on a commutator segment so that the claw is embedded in the molding resin to thereby increase the boding strength against the centrifugal force. However, it is difficult to form such a claw without increasing a size of the commutator because the claw must be formed at a radial inside of the segment where an available space is limited. In other words, it is unavoidable to increase the commutator size to form such a claw having sufficient mechanical strength.
Another proposal is made in JP-A-1-283043. It proposes to form a projection extending to an opposite side of a riser to which armature coil ends are connected in addition to the claw embedded in the molding resin. However, the projection formed at an opposite side of the riser is not able to sufficiently increase the bonding strength against the centrifugal force. This is because a larger centrifugal force is applied to the riser that has a larger volume than the other portion of the segment. If the segments are peeled off from the bonding resin, brushes contacting the segments will abnormally wear and normal commutation will not be performed.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved commutator that has a sufficient strength against a centrifugal force without increasing the size of the commutator. Another object of the present invention is to provide a starter for cranking an internal combustion engine, in which a direct current motor having the improved commutator is used.
A direct current motor is composed of a stator for generating a magnetic field therein, a rotor having armature coils and rotating inside the stator, a commutator connected to the rotor, brushes slidably contacting the commutator for supplying electric current to the armature coils, and other associated components. All the components of the direct current motor are contained in a motor housing. The commutator is composed of plural commutator segments arranged in a circular shape and an insulation bond for boding the commutator segments into a single body of the commutator. The insulation bond is formed by molding a resin material.
Each commutator segment is made of copper and includes an elongate portion contacting the brushes, a portion for connecting ends of the armature coils thereto, an anchor portion embedded in the insulation bond, and a projecting portion also embedded in the insulation bond for increasing a bonding strength against a centrifugal force applied to the commutator. The connecting portion extends in a radial direction from one axial end of the contacting portion. The anchor portion is formed at a radial inside of the contacting portion, and the projecting portion extends from a foot of the connecting portion to the axial direction which is opposite to the direction to which the contacting portion extends.
To further increase the bonding strength, a depressed portion may be formed at one axial end of the anchor portion so that a circular band is disposed in a circular groove formed by depressed portions of plural commutator segments. The circular band is also embedded in the insulation bond. The depressed portions may be formed at both axial ends of the anchor portion so that two circular bands are disposed in the circular grooves formed at both axial ends.
Since not only the anchor portion but also the projecting portion is embedded in the insulation bond, the commutator segments are firmly bonded together with the insulation bond. Therefore, a sufficient bonding strength that endures a large amount of centrifugal force applied to the commutator is secured without enlarging the size of the commutator. The direct current motor rotating at a high speed can be realized without increasing the size. Also, a compact high speed starter for cranking an internal combustion engine is realized by employing the direct current motor according to the present invention.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.